On the drag reduction mechanism in a lubricated turbulent channel flow

Turbulent flow in a lubricated plane channel has been considered. Direct numerical simulations showed that a substantial drag reduction can be achieved when thin films are lubricating the channel walls. The wall-normal velocity is suppressed inside the lubricating film, whereas planar fluid motions prevail. The bulk fluid is no longer exposed to no-slip at the solid surfaces, but the turbulence statistics and the coherent near-wall structures nevertheless closely resemble those observed in a dry channel. It is concluded that the interface between the bulk-fluid and the more viscous lubricating layer acts as an insulator which hinders the intense downwash of high-speed fluid, caused by the coherent vortices, to hit the solid surface.